A large percentage of the electrical power generated in the United States of America is created in coal combustion power plants. The bulk of worldwide electricity production similarly relies on coal as a primary energy source. It is likely that coal will remain a primary energy source in the foreseeable future given the long term environmental concerns with the storage of waste from nuclear energy generation operations, and the inefficiencies associated with solar powered electrical generation. In addition vast worldwide coal reserves exist sufficient for at least 200 years of energy production at current rates.
There is and will continue to be, however, a high demand to reduce the emissions of pollutants associated with coal fired electrical energy generation, and to increase the overall efficiency of the coal fired generation process. The monitoring of the O2 and other gas levels within a combustion chamber or power plant furnace is one key component of efficiency monitoring and control. Traditionally, in power plants and other industrial combustion settings the efficiency of the combustion process and the level of pollution emission have been determined indirectly through measurements taken on extracted gas samples with techniques such as non-dispersive infrared (NDIR) photometry. Extractive sampling systems are not particularly well suited to closed loop control of a combustion process since a significant delay can be introduced between the time of gas extraction and the ultimate analysis. In addition, extractive processes generally result in a single point measurement which may or may not be representative of the actual concentration of the measured species within what can be a highly variable and dynamic combustion process chamber.
Laser based optical species sensors have recently been implemented to address the concerns associated with extraction measurement techniques. Laser based measurement techniques can be implemented in situ and offer the further advantage of high speed feedback suitable for dynamic process control. A particularly promising technique for measuring combustion gas composition, temperature and other combustion parameters is tunable diode laser absorption spectroscopy (TDLAS). TDLAS is typically implemented with diode lasers operating in the near-infrared and mid-infrared spectral regions. Suitable lasers have been extensively developed for use in the telecommunications industry and are, therefore, readily available for TDLAS applications. Various techniques of TDLAS which are more or less suitable for the sensing and control of combustion processes have been developed. Commonly known techniques are wavelength modulation spectroscopy, frequency modulation spectroscopy and direct absorption spectroscopy. Each of these techniques is based upon a predetermined relationship between the quantity and nature of laser light received by a detector after the light has been transmitted through a combustion process chamber and absorbed in specific spectral bands which are characteristic of the gases present in the process or combustion chamber. The absorption spectrum received by the detector is used to determine the quantity of the gas species under analysis plus associated combustion parameters such as temperature.
For example, Von Drasek et al., United States Patent Application Serial Number 2002/0031737A1, teaches a method and apparatus of using tunable diode lasers for the monitoring and/or control of high temperature processes. Von Drasek features the use of direct absorption spectroscopy to determine the relative concentration of numerous combustion species, temperature and other parameters. Calabro, U.S. Pat. No. 5,813,767, teaches a similar system for monitoring combustion and pollutants developed in a combustion chamber. Calabro utilizes an indirect spectroscopy technique wherein observed Doppler broadening of the shape of an absorption feature serves as the basis for temperature analysis.
Teichert, Fernholz, and Ebert have extended the use of TDLAS as a known laboratory analysis technique to a workable field solution suitable for the sensing of certain combustion parameters within the furnace of a full sized coal fired power plant. In their article, “Simultaneous in situ Measurement of CO, H2O, and Gas Temperature in a Full-Sized, Coal-Fired Power Plant by Near-Infrared Diode Lasers,” (Applied Optics, 42(12):2043, 20 Apr. 2003) the authors present a successful implementation of direct absorption spectroscopy at a coal fired power plant and discuss certain technical challenges resulting from the extremely large scale and violent nature of the coal burning process. In particular, typical coal fired power plants have combustion chamber diameters of 10-20 meters. The plants are fired by pulverized coal, which results in a combustion process which both obscures the transmission of laser light because of the high dust load and which is extremely luminous. In addition, various strong disturbances are found under power plant combustion conditions. The overall transmission rate of light through the process chamber will fluctuate dramatically over time as a result of broadband absorption, scattering by particles or beam steering owing to refractive-index fluctuations. There is also intense thermal background radiation from the burning coal particles which can interfere with detector signals. The environment outside of the power plant boiler also makes the implementation of a TDLAS sensing or control system problematic. For example, any electronics, optics or other sensitive spectroscopy components must be positioned away from intense heat, or adequately shielded and cooled. Even though the implementation of a TDLAS system is extremely difficult under these conditions, TDLAS is particularly well suited to monitor and control a coal combustion process. A comprehensive discussion of the use of TDLAS to monitor and control a combustion process is contained in commonly assigned and copending PCT Application Serial Number PCT/US04/010048, filed Mar. 31, 2004, entitled METHOD AND APPARATUS FOR THE MONITORING AND CONTROL OF COMBUSTION, which application is incorporated herein by reference in its entirety.
Typically, the electronic, optical, and other sensitive spectroscopy components which must communicate with the interior of a combustion chamber are associated with a special opening into the combustion chamber. This opening or port will often feature a quartz, fused silica, or other window fabricated from a transparent material which is stable at the extremely high temperatures associated with the interior of the combustion chamber. Alternatively, the opening may not include a transparent window. In either case, the opening in a typical coal fired power plant must transverse the furnace wall and may be about 18 inches long. As described above, the interior of the combustion chamber is an extremely hostile environment full of pulverized coal, ash, and other particulate matter. Thus, there is a tendency for the opening or port to become clogged or partially blocked with ash and other particulate matter.
Port blockage can be addressed by flowing purge air through the port. The purge air may be constantly flowed through the port from a captive purge gas supply or, more commonly, ambient air from outside of the combustion chamber may be utilized Typically, purge air will thus include significant amounts of O2 relative to the O2 levels within the combustion chamber.
It is useful to monitor the O2 or other gas levels as part of the TDLAS monitoring and control of a combustion process. The introduction of O2 containing purge gas significantly complicates this measurement. Depending on the location in the furnace where a measurement is taken, the purge gas may include anywhere from 30% to 60% of the total O2 present along a TDLAS path. Only the O2 in the furnace is of interest for proper combustion control. Similarly any absorption spectroscopy measurement of a quantity of gas is complicated if a second quantity of the gas is also present in the measurement path. Thus a need exists for a method to accurately quantify the effect of purge gas O2 on the desired combustion chamber O2 measurement. The present invention is directed to overcoming one or more of the problems discussed above.